Microsoft blamed Delta Air Lines on Tuesday for its dayslong struggle to recover from a global cyber outage that led it to cancel more than 6,000 flights.

A software update last month by global cybersecurity firm CrowdStrike triggered system problems for Microsoft customers, including many airlines. But disruptions subsided the next day at other major U.S. carriers while persisting at Delta.

Microsoft said its preliminary review suggested that Delta, unlike its competitors, apparently had not modernized its IT infrastructure.

Delta, however, said it has invested billions of dollars in IT capital expenditures since 2016, in addition to the billions it spends every year in IT operating costs.

“Delta has a long track record of investing in safe, reliable and elevated service for our customers and employees,” a company spokesperson said.

The flight disruptions stranded hundreds of thousands of travelers and are estimated to cost the Atlanta-based airline $500 million. Delta is also facing an investigation from the U.S. Transportation Department for the disruptions.

It has hired prominent litigator David Boies of Boies Schiller Flexner, known for high-stakes business cases, to seek damages from both CrowdStrike and Microsoft.

Last week, Delta’s CEO Ed Bastian told CNBC that while the airline heavily relies on the two tech companies, they had failed to deliver an “exceptional service.” In the interview, he also said that Microsoft had the “most fragile platform.”

In a letter, Mark Cheffo, a lawyer for Microsoft, called the airline’s comments “incomplete, false, misleading, and damaging to Microsoft and its reputation.”

Cheffo said Microsoft’s software had not caused the CrowdStrike incident, but the tech giant immediately offered to assist Delta at no charge. Its CEO Satya Nadella emailed Bastian, but never got a reply, he added.

Microsoft said its employees repeatedly offered to help Delta following the outage, but the U.S. carrier turned them down. It accused the airline of using services of other technology providers for its crew-tracking and scheduling system and alleged it as a likely reason for declining its help.

Cheffo said Microsoft will “vigorously” defend itself if Delta files a lawsuit.

CrowdStrike has also rejected Delta’s claim that it should be blamed for flight disruptions. The cybersecurity firm said its CEO had personally reached out to Bastian to offer onsite assistance, but received no response.

(Reporting by Rajesh Kumar Singh; Additional reporting by David Shepardson; Editing by Franklin Paul and Josie Kao)